Project Summary / Abstract Nature uses protein allostery to control long-range electron flow in chemical reactions essential for life. Metals and metal-clusters are often key components facilitating this electron transfer. A known allosteric method for gating electron flow in metalloenzymes is alternating-site reactivity, wherein reactivity switches from one half of the enzyme to the other upon conformational changes. Oxidoreductases, with a core α2β2 heterotetrameric structure, are iron-sulfur cluster metalloenzymes that employ alternating-site reactivity and can serve as an invaluable model system to define the structural motifs and mechanisms required for metalloenzyme allosteric regulation. In line with the NIH NIGMS Pharmacology, Physiology, and Biochemical Chemistry mission of “improving molecular level understanding of fundamental biological processes and discovering approaches to their control”, we propose to study two such oxidoreductases, the nitrogenase-like dark-operative chlorophyllide oxidoreductase (DPOR) and chlorophyllide oxidoreductase (COR), to identify how long-ranged correlated motions contribute to alternating- site reactivity using structural biology tools. We will employ electron microscopy and small-angle X-ray scattering techniques to examine enzyme conformational intermediates with asymmetrical conformations.